1. Field of the Invention
The present invention relates to an adhesive, particularly an extrudable adhesive, for bonding a polar substrate particularly a metal foil such as aluminum foil to a non-polar substrate particularly a polymer film such as polyethylene film.
2. Description of Related Art
Plastic films, particularly polyethylene films, laminated to, coated on, or co-extruded with metal foil, particularly aluminum foil, have found utility in packaging and other applications such as cable shielding. The plastic film and the metal foil may be combined with other materials to form a structure having a plurality of layers, each layer having a specific purpose. Packaging laminates, for example, often comprise a plurality of layers. Such a packaging laminate might have a configurationally rigid core layer of paper or paperboard, an outer liquid-tight layer of polyethylene and an oxygen gas barrier mid-layer of aluminum foil and, optionally, other layers depending on application needs.
To form an effective laminate, it is important that good bonding strength or intimate integrity between the layers be achieved for most applications. For materials that do not normally adhere well to each other, an adhesive binder or tie layer is used to bond adjacent layers together.
Low density polyethylene (LDPE) has become the mainstay of the foil extrusion coating industry. It is low in cost, processes well, and has adequate normal adhesion to metal foils for many applications. For demanding applications, where durability of bond strength in the presence of aggressive products, good heat seal characteristics, and excellent food adhesion are desired, LDPE alone has limitations.
Ethylene xcex1,xcex2 ethylenically-unsaturated carboxylic acid copolymers, particularly copolymers of ethylene and acrylic or methacrylic acid (E/AA and E/MAA), optionally partially neutralized, (available from the DuPont Company under the tradenames, xe2x80x9cNucrelxe2x80x9d, xe2x80x9cBynelxe2x80x9d, and xe2x80x9cSurlynxe2x80x9d) bond particularly well to metal foils. The acid groups on such an ethylene acid copolymer resin (ACR) provide bonding sites to the basic oxides on the foil surface. By increasing the acid level, adhesion to the foil is increased. The ethylene acid copolymers, however, are more costly than the LDPE and don""t bond well with LDPE particularly with increasing acid levels.
There thus remains a need for an adhesive tie layer that will intimately bond to polyethylene, particularly LDPE, and to metal foil. There is a need to form a laminate that is less costly than a foil/ethylene acid copolymer laminate and that exhibits a superior durability of bond strength in the presence of aggressive products.
The present invention relates to an adhesive, particularly an extrudable adhesive that has improved adhesion to both polar substrates such as metal foil and to non-polar substrates such as polyethylene. It particularly relates to an adhesive that produces a bond between metal foil and polyethylene that is durable in the presence of aggressive products such as ketchup, flavoring oils, juices and the like.
The adhesive of the present invention is an ethylene acid copolymer (ACR) blend consisting essentially of a high-acid, high-melt index (MI) acid copolymer blended with an acid copolymer that has both a lower acid level and a lower MI than the high-acid copolymer
The present invention also relates to a laminate comprising a polar substrate such as a metal foil and a non-polar substrate such as polyethylene film bonded to each other with a tie layer of the adhesive of the present invention, and to the use of such a laminate in packaging and cable shielding applications.
As summarized above, the present invention relates to an adhesive, particularly an extrudable adhesive, to a laminate comprising a polar substrate, particularly a metal foil such as aluminum foil, and non-polar substrate, particularly a polyethylene film bonded by a tie-layer of the adhesive, and to uses of the laminate.
The laminates of the present invention have a plurality of layers, at least one of which being a non-polar substrate being bonded to at least one layer that is a polar substrate by a tie layer of adhesive film.
The thickness of the laminates of the present invention depends on the application and the substrates used. Typical packaging laminates preferably have a thickness of about 0.1 mil (2.5 micrometers) to about 10 mil (250 micrometers), more preferably of about 0.5 mil (12.5 micrometers) to about 2 mil (50 micrometers). Cable shield laminates would be substantially thicker. A paper substrate typically will be relatively thick (about 1 to about 10 mil (25 to 250 micrometers), films will be relatively thin (about 0.2 to about 2 mil (5 to 50 micrometers), and foils will typically be thinner still (about 0.2 to 2 mil (5 to 50 micrometers).
The non-polar substrate preferably is a polyethylene film. The polyethylene film can be selected from low density polyethylene (LDPE), linear low density polyethylene (LLDPE), or high density polyethylene (HDPE). It may be made by any available process known for making such polyethylene including high pressure gas, low pressure gas, solution and slurry processes employing conventional Ziegler-Natta, metallocene, and late transition metal complex catalyst systems.
The preferred polar substrate is a metal foil, more preferably aluminum foil. Other substrates include metalized substrates such as metalized polypropylene, paper, polyester, nylon, and ethylene vinyl alcohol. The polar substrate may be surface treated by means known in the art such as corona treatment to improve the adhesion, but such treatment is not required in the present invention and preferably is not used in the case of foils and metalized substrates.
Film substrates may be primed with various primers known in the art, particularly PEI and polyurethane primers, before applying the adhesive tie layer of the present invention.
The adhesive employed in the tie layer is a blend of ethylene-xcex1,xcex2 ethylenically-unsaturated C3-C8 carboxylic acid copolymers (ACR""s). It particularly is a blend comprising about 5 to about 95, preferably about 15 to about 85, and more preferably about 15 to about 40, parts by weight High-Acid, High-MI ACR and about 5 to about 95, preferably about 15 to about 85, and more preferably about 60 to about 85, parts by weight Low-Acid, Low-MI ACR. Preferably, the Low-Acid, Low-MI ACR is the major component and the High-Acid, High-MI ACR is the minor component. More than two ACR""s may be included so long as at least one is a High-Acid, High-MI ACR and one is a Low-Acid, Low-MI ACR and the other copolymers do not compatibilize the High-Acid, High-MI and Low-Acid, Low-MI ACR""s in a way that prevents the High-Acid, High-MI ACR from moving to the surface away from a non-polar substrate when the adhesive is co-extruded with the non-polar substrate.
Both the High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR (as well as other ACR""s that may be in the blend) are ethylene-xcex1,xcex2 ethylenically-unsaturated C3-C8 carboxylic acid copolymers, optionally having at least one softening co-monomer that is co-polymerizable with ethylene. Acrylic and methacrylic acids are preferred acid co-monomers. The softening co-monomer can be an alkyl acrylate selected from the group consisting of n-propyl-, n-butyl-,i-butyl-, n-octyl-, 2-ethylhexyl-. and 2-methoxyethyl-acrylates. The preferred alkyl acrylates are n-butyl-, i-butyl-, 2-ethylhexyl-, and 2-methoxyethyl-acrylates. The softening co-monomer can also be an alkyl vinyl ether selected from the group consisting of n-butyl, n-hexyl, 2-ethylhexyl-, and 2-methoxyethyl-vinyl ether. The preferred alkyl vinyl ethers are n-butyl vinyl ether and n-hexyl vinyl ether.
Preferably the High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR are not neutralized. Either or both may be partially neutralized, however, so long as the resulting un-neutralized acid levels and the melt indices of the resulting partially neutralized ACR""s meet the limitations set forth for the non-neutralized ACR""s. Preferably, low levels of neutralization can be employed through the addition of sources of cations such as zinc, sodium, magnesium and the like. Preferred low levels are less than 500 parts per million (more preferably between about 100 and 200 or 300 ppm) based on the total weight of the High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR in the blend.
Useful High-Acid, High-MI ACR and Low-Acid, Low-MI ACR copolymers are available from DuPont under the Trademarks NUCREL(copyright) and BYNEL(copyright). The High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR are more fully described as follows:
High-Acid, High-MI ACR
High-Acid, High-MI ACR preferably is an ethylene acrylic acid or an ethylene methacrylic acid copolymer. The acid content and the MI need not be xe2x80x9chighxe2x80x9d in an absolute sense; they must only be high in the sense of being higher than the acid content of the Low-Acid, Low-MI ACR and at least as high or higher than the MI of the Low-Acid, Low-MI ACR.
Preferably the acid percentage of the High-Acid, High-MI ACR is 3 weight percentage points or more higher than that of the Low-Acid, Low-MI ACR (that is, for example, for a Low-Acid, Low-MI ACR acid level of 9 weight percent (wt. %), the High-Acid, High-MI ACR acid level would be 12 wt. % or higher). Preferably the difference is 3 to 15, more preferably 3 to 6 weight percentage points. Preferably the weight percent acid in the High-Acid, High-MI ACR is about 7 to about 25, more preferably about 10 to about 20 wt. %.
The MI (measured in accord with ASTM D-1238, condition E, at 190xc2x0 C., using a 2160 gram weight) of this High-Acid, High-MI ACR must be at least as high as or higher than the MI of the Low-Acid, Low-MI ACR. The preferred MI of the High-Acid, High-MI ACR is about 7 to about 1000, more preferably about 10 to about 100 or about 10 to about 60 grams/10 minutes.
Low-Acid Low-MI ACR
Low-Acid, Low-MI ACR preferably is an ethylene acrylic acid or an ethylene methacrylic acid copolymer. The acid content need not be xe2x80x9clowxe2x80x9d in an absolute sense; it must only be lower than the acid content of the High-Acid, High-MI ACR.
Preferably the weight percent acid in the Low-Acid, Low-MI ACR is about 1 to about 22, more preferably about 4 to about 15 or-about 7 to about 12 wt. %.
The M of the Low-Acid, Low-MI ACR must be at least as low as and preferably lower than the MI of the High-Acid, High-MI ACR. Preferably the MI is about 0.1 to about 20, more preferably about 2 to about 14 or about 2 to about 9 grams/10 minutes.
Blend
The High-Acid, High-MI ACR and the Low-Acid, Low-MI ACR are selected to yield a resultant MI of the blend in a range that can be processed in co-extruding equipment, particularly with LDPE. The resultant MI preferably should be in the range of about 4 to about 20, more preferably about 6 to about 14 grams/10 minutes. The resultant acid level preferably is more than 1 wt. %, preferably about 7 wt. % or more, preferably about 7 to about 12 wt. %. These percentages are preferably methacrylic acid equivalents.
Process For Making Adhesive Tie Layer
The blend of High-Acid, High-MI ACR and Low-Acid, Low-MI ACR according to the invention can be made by melt blending the polymers under medium to high shear conditions, for example in a single-screw or a twin-screw extruder. The High-Acid, High-MI ACR and Low-Acid, Low-MI ACR may first be combined with one another (e.g., in a pellet blend), or they may be combined with one another via simultaneous or separate metering of the various components to the extruder.
Process For Making Laminate
The process for making the laminate preferably is a co-extrusion coating process well known in the art such as the one described below.